For servicing works on electrical systems, it must be determined whether the electrical system is de-energized before said works can be carried out. Simply disconnecting the electrical system from the electricity supply grid or the electrical energy supply is not a reliable enough basis for determining that the circuit is de-energized since electrical energy may still be stored in electrical energy stores, e.g. capacitors. In addition, other electrostatic charges which may lead to a low residual voltage in the electrical system are also conceivable. Yet voltage measurement on the electrical system does not guarantee the system is de-energized either, even if no voltage is detected. Due to possible contacting faults between the measurement contacts of a measurement instrument and the equipment under test, a de-energized state may be erroneously indicated, or a low residual voltage may be indicated despite the distinct possibility of a (high) voltage still being applied at the equipment under test. In addition, an electrostatic charge in the electrical system may lead to a low voltage being displayed due to the high-resistance input of a voltage measurement device. The display of a very low voltage may thus also originate from the system due to faulty contacting or a residual voltage actually being present.
Therefore, to reliably determine whether an electrical system is de-energized in order to be able to reliably carry out further work on the electrical system, the contacting of the measurement contacts of the measurement instrument must be checked. In the event of faulty contacting, a relatively low voltage or in the worst case scenario no voltage at all could be measured as being applied between the measurement contacts; this is a potential danger for people who will be carrying out work on the electrical system. However, conventional measurement instruments do not offer this function. Previously, therefore, contacting checks have been carried out visually. In other words, the user checks visually whether the measurement contacts properly contact the equipment under test. However, in many applications, e.g. in vehicles having electrified propulsion, the confined spaces or position of the measurement contacts mean visual verification of the contacting cannot be carried out or cannot be carried out unequivocally. Moreover, conclusions on actual electrical contacting cannot always be made on the basis of visual contacting.
It would be conceivable to use the resistance measurement, often provided for in measurement instruments, for the purposes of a contacting check. In this case, a current or voltage source is integrated in the measurement instrument and conclusions can be drawn on a contacted measurement contact by measuring the current flow via the measurement contacts of the measurement instrument. In this manner, however, only purely ohmic resistance and additionally only low-ohmic resistance can be detected. A contacting error, which can be equated to a high-ohmic resistance, cannot be reliably and unequivocally determined in this manner. In addition, a high residual voltage can lead to destruction of the measurement instrument if said instrument is in resistance measurement mode.